I agree that Marathon and Liberty are a couple of the top Prevost converters. However, if you really want a class act, don't forget
American Carriage. They make a quality Prevost conversion although in smaller quantities than Marathon and Liberty. For design and engineering of a Prevost conversion, I would pick American Carriage first, then Marathon, then Liberty and Parliament tied. However, I recognize that each persons needs and wants are different so your mileage may vary.
I still believe that dollar for dollar, Newell builds a better coach.